The present invention relates generally to digital communication systems, and more particularly to a hybrid switching and transmission system in which a designated portion of digital information is assignable to a permanent slot and to more than one additional temporary slots as necessary for efficient use of system resources for integrated traffic.
Hybrid switching systems of the prior art use a specialized framing technique to integrate conventional voice and data traffic, in recognition that the hybrid integration principle is based on a sharing of the same transmission carrier to incorporate both circuit (voice) and packet (data) traffic. A multi-bit envelope (or frame) is constructed which is transmitted through the carrier, with variations achieved by varying frame construction. Toward that end, the prior art has moved toward variations of the slotted envelope network (SENET) concept discussed by Coviello et al. in their presentation on "Integration of circuit/packet switching by a SENET concept" at the National Telecommunications Conference in New Orleans in 1975. The variants proposed thus far are characterized by either a fixed or a variable envelope length. According to the SENET concept, frames are divided into slots for voice and data. Slot sizes are variable to handle various bit rate traffic, and slots between circuit-switched channels are separated by a boundary.
A variety of SENET versions have been proposed in the past; for example, by Weinstein et al., "Data traffic performance of an integrated circuit- and packet-switched multiplex structure," IEEE Transactions on Communications, Com-28, No. 6, 873-878, June 1980; by Bially et al., "Voice communications in integrated digital voice and data networks," IEEE Transactions on Communications, Com-28, No. 9, 1478-1490, September 1980; by Fisher et al., "A model for evaluating the performance of an integrated circuit- and packet-switched multiplex structure," IEEE Transactions on Communications, Com-24, No. 2, 195-202, February 1976; and by Gerla et al., "Bandwidth allocation and routing in ISDN's," IEEE Communications Magazine, 22, 2, 16-26, February 1984. These proposals include fixed boundaries, movable boundaries, and fixed/movable boundaries with silent activity detection (SAD).
The version with dynamically movable boundaries and SAD implementation offers good bandwidth utilization, but suffers certain disadvantages. Among the latter are additional overhead and additional processing delays resulting from the need to inspect "busy bit" in each slot, as discussed, for example, in the aforementioned Gerla et al. publication. The dual nature of the SENET's frame simplifies the interfaces with existing separate circuit and packet networks.
Alternatives to the dual frame are a fixed-length packet frame or a non-fixed burst frame. Fixed-length packet frames are the foundation of fast packet switching, as described by Turner, "Design of an integrated services packet network," Proceedings of the Ninth Datacomm Symposium, ACM SIGCOMM Computer Communications Review 15, No. 4 124-133, September 1985. As a universal carrier, fixed-length packets provide for transmission of a viariety of multimedia traffic components.
On the other hand, non-fixed burst frames are the foundation of burst switching technology, which is based on the concept that bandwidth should be allocated to the voice/data call only during the talk spurt or data message. In essence, burst switching technology recongizes that gaps exist between signals, and that in the case of a voice signal, an active period of voice transmission is followed by a gap or silence. As soon as the switching system detects the gap, it immediately places the bandwidth in a common pool for availability to other users. From that standpoint, burst switching technology is a statistical multiplexing system.
A non-fixed burst frame is a variable digital bit stream consisting of three elements, namely, (1) a header containing routing and control information, (2) the contents of the burst consisting of message information, which may be digitized voice, user data, or control data, and (3) an "end-of-burst" code. The configuration of the non-fixed burst frame is shown in FIG. 1. The duration of the active voice period will vary, and, hence, the burst length is variable. The variable length of the burst is a significant feature of burst switching technology. The same situation occurs with data transmissions as that with voice transmissions. The data packet or block is put into a slot, and as soon as the sender completes its transmission, that portion of the bandwidth is given to another user.
Because of burst switching technology, different sources of message information (voice or data) can be combined on the same transmission medium frame, T1. By way of example, and with reference to FIG. 2, assume that T1 has twenty-four channels, and that source #i is active. Upon recognition that source #i is active, the T1 frame is looked at and it is found that channel #n is free. Source #i is the assigned to that channel and retains it for the duration of the burst. Consequently, each T1 frame the information is sent byte by byte from source #i to the receiver until, after the last byte has been transmitted, channel #n becomes free.
Although other levels of burst switching technology involve switching architecture and network structure, the principle constituting the foundation of that technology is relatively simple: that is, recognize the burst transmission, assign a channel, and retain the channel until the burst is ended. For the sake of simplicity of the present discussion, only point-to-point transmission is considered. Channels are not permanently assigned to particular sources. The next source that becomes active during a period of transmission by source #i on channel #n will simply occupy whatever other channel is available. All statistical multiplexers operate on this basis.
Dual SENET, fixed-length packet, and non-fixed burst frames have been developed as a foundation for integrated switching systems. They reflect a different, and sometimes contradictory set of objectives which, historically, have been established for integrated transmission of conventional voice, interactive data, and control information.
Statistical multiplexing with limited resource management has heretofore been the most popular approach for the packet side of the hybrid switching/transmission. The designated portion of the digital information, block or burst, which is assigned to the proper slot cannot be reassiagned to an additional slot until the end of the block or burst.